Peavey KB300 only hums

[Sitruc99]

Greetings all, new to the forum. Hope I have posted in the right area. I have a Peavey KB300 keyboard amp given to me. I'm a drummer using it for edrums. He said after moving it over some very rough Terran, the amp just hums when turned on. I have been searching this forum and the internet and done some work on it.
I have replaced the 2 filter caps with the 63 volt 4300 ones and 1 resister that looked like it "leaked".
Didn't help. The speaker sucks in and hums when turned on. I do get signal from the line out when I plug something in up front.
I saw where a guy was getting DC voltage to his speaker online. Sure enough, when I checked, I am getting about 38 volts DC from the 2 pins that go to the horn and wolfer.
I have a KB100 now. I plugged that amp head into the 300 speakers, just for a short test because the impedance is different. The speakers work very well, no problems.
I have checked the board for bad solder joints and anything that might look bad.
I'm no electronics guy, but if anyone could lead me in the right direction it would be much appreciated! Thanks!!!

[g1]

Don't run it anymore with the speaker connected until you have fixed the DC on output problem.
If it really was working just before the bump and quit working after, chances are there is a bad solder connection somewhere in the power amp section.

[Enzo]

This ^^^^^

Turn it OFF, you are going to kill your speaker. The amp is making DC on the output.


Sometimes bad solder joints are visually apparent, but plenty of bad solder joints LOOK just fine. Many things in the power amp can do this.

[g1]

Just to be clear, you can still run it with speaker disconnected if you need to for troubleshooting.

[Sitruc99]

Thanks for the responses. I never ran it long at all, thankfully! I will check the circuit board again. Go through it real good for soldering.

[Enzo]

Check the output transistors for shorted.

[Sitruc99]

I should add that the comp light comes on also when turned on.

[The Dude]

Schematic link for those wanting to play along.

https://music-electronics-forum.com/...0&d=1397049266

[Jazz P Bass]

I took notice that the schematic is the 'No XLR' version.

Here is the one with an XLR input jack on the front panel: Peavey-KB300-sch.zip

[Sitruc99]

I tested the 4 output transistors on the board and got different readings so I pulled each one. Here's what I got.
B to E in-.038 out-.478
B to C in-.408 out-.478
E to B in-.038 out -OL
C to B in-1&2-1.293 3&4-.587 out-OL
C to E in-1&2-1.277 3&4-.608 out-OL
This was done on the diode test on my meter. I am still am getting 38 volts DC on the speaker jack. I also went thoroughly through the board (at least the larger components on the filter caps side half of the board) and resoldered.
Still 38 volts DC.
Not sure why I did this but I removed the 14 pin IC and tested the speaker jack. It measured 28 volts DC then.
Put everything back threw a bunch of water on it (just kidding). Still getting 38 volts DC. The comp light did go out with IC removed. It also goes out with the plug removed (2 red wires and a white jumper). Anyway, stop rolling your eyes and help a drummer out! Thanks

[The Dude]

You might be better off taking some voltage measurements. The output transistors seem fine. Let's start with Q7 and Q11. Please list the voltages on each transistor E,B,&C.

Edit: I guess it depends on which schematic you're looking at as far as designation numbers. Anyway, just pick two output transistors- one that connects to the positive rail and one that connects to the negative rail.

[Enzo]

Yes, PLEASE tell us which KB300 you have, the 1983 design, or the 1989 design with the XLR jack on the front.

[Sitruc99]

Sorry guys. I have the earlier version with NO xlr in the front. I will try and check some voltages asap. Going back to work at the casino here in Black Hawk CO, holidays bring out the gamblers. Thanks again for the guidance.

[Sitruc99]

Looks like I have the following voltages.
Q6&7 - E+38.8 C+39.6 B+40.7
Q10&11 - E-40.4 C+40.1 B-40.7

[Enzo]

So Q10,11 ar not being turned on. So explore Q9 and also make sure the dual diode CR9 and associated R36 is not open.